Implement this in addition toConfigIncluder if you want to
support inclusion of files with the include file("filename") syntax.
If you do not implement this but do implement ConfigIncluder,
attempts to load files will use the default includer.
Parses another item to be included. The returned object typically would
not have substitutions resolved. You can throw a ConfigException here to
abort parsing, or return an empty object, but may not return null.